Fix verified doubles when their const has already been stubbed.

Fixes #823.
Remove unnecessary captured block.

Capturing blocks isn’t free, and `yield` (which
we are already using) performs better than
capturing the block and using `block.call`.
Fix memoization to properly memoize nil.
Merge pull request #824 from rspec/fix-instance_double-when-class-const-stubbed

Fix verified doubles when their const has already been stubbed.
allow verifying doubles to have custom names
add class documentation about named verifying doubles
mention naming verifying doubles in README